1. Field of the Invention
The invention is generally related to the field of wellbore logging instruments. More specifically, the invention is related to devices for measuring the internal diameter of a wellbore to enable more reliable determination of whether a wellbore fluid sample taking instrument is likely to be properly placed in hydraulic communication with earth formations from within the wellbore.
2. Description of the Related Art
Wireline formation fluid sample taking instruments are used to extract samples of connate fluid from earth formations penetrated by a wellbore. Generally, these instruments include a tubular probe which is extended from the housing of the instrument and is hydraulically sealed against the wall of the wellbore. The probe is then selectively placed in hydraulic communication with a pump or a sample chamber, or some similar combination of elements used to withdraw fluid from within the pore spaces of the earth formation. See for example, U.S. Pat. No. 4,507,957 issued to Montgomery et al which describes one type of fluid sample taking instrument.
Wireline formation fluid sample taking instruments are susceptible to failure of a sealing element (xe2x80x9cpackerxe2x80x9d) surrounding the probe if the wellbore wall is not smooth, or if the wellbore is substantially enlarged beyond the diameter of a drilling bit used to drill through the earth formations. The packer may also fail to seal properly if the instrument is not properly centered in the wellbore and is put into skewed contact with the wellbore wall. In these cases, when the system operator causes the instrument to withdraw fluid, fluid disposed in the wellbore itself can be drawn across the face of the packer and enter the probe, thereby making the sample unrepresentative of the connate fluid in the earth formation. The system operator can generally determine whether his type of seal failure has occurred by observing measurements of the fluid pressure in the probe. Rapid increase in pressure to the same pressure as the hydrostatic pressure of the fluid in the wellbore typically indicates packer failure. If the cause of the packer failure is roughness of the wellbore wall, slight movement of the instrument along the wellbore may result in a successful reattempt at withdrawing a formation fluid sample.
However, the system operator may not be able to determine whether the instrument is disposed in a part of the wellbore in which the wellbore diameter is substantially enlarged past the drilling bit diameter, or even enlarged past the operating diameter range of the fluid sample taking instrument. The system operator also may not be able to determine if the instrument is not well centered in the wellbore where the packer is placed into skewed contact with the wellbore wall. It is known in the art to use xe2x80x9ccaliperxe2x80x9d logs to estimate whether the instrument is disposed in such an enlarged part of the wellbore. Caliper logging instruments which can be included with other types of wellbore logging instruments are well known in the art. See for example, U.S. Pat. No. 4,432,143 issued to Moriarty et al or U.S. Pat. No. 4,559,709 issued to Beseme et al. Generally speaking, the caliper logging instruments known in the art include an xe2x80x9carmxe2x80x9d or other member which is placed in continuous contact with the wellbore wall. The arm is coupled to some type of position sensor to determine the amount of lateral extension of the arm from the instrument housing.
Using caliper logs which have been measured by instruments other than the formation sample taking instrument is not always conclusive as to whether the wellbore diameter at the selected formation sample depth is greater than the extension range of the formation fluid sample taking instrument. As is known in the art, the wellbore may xe2x80x9cwashxe2x80x9d or otherwise become enlarged past the diameters recorded by the earlier-run caliper log by the time the fluid sample taking instrument is to be run in the wellbore. It is therefore desirable to include a caliper instrument along with the formation sample taking instrument.
The typical wellbore caliper logging instrument is designed, however, for measurement while the instrument is being moved along the wellbore. Further, the typical wellbore caliper logging instrument includes complex linkages to couple the arm to the position sensor, because the sensor itself must generally be located inside the instrument housing to avoid destruction by the fluid in the wellbore. See U.S. Pat. No. 4,559,709 issued to Beseme et al for example. It has proven impracticable to include an arm-type caliper, or any other type of caliper, at the location of the probe in a formation fluid sample taking instrument, principally because the sample taking instrument includes a xe2x80x9cback upxe2x80x9d shoe or similar device which is hydraulically extended from the instrument housing at a circumferential location opposite to the probe, to force the probe into contact with the wellbore wall under very high lateral force. The complex hydraulic components necessary to actuate the typical back up shoe have made including a position sensor at the location of the back up shoe and probe very difficult.
The invention is an apparatus and method for measuring the internal diameter of a wellbore used with a wireline formation sample taking instrument. The apparatus includes an hydraulically actuated probe and back up shoe for selective engagement with the wall of a wellbore, an hydraulic pump and selectively controllable valves for selectively controlling the extension and retraction of the probe and back up shoe, and an hydraulic fluid reservoir to supply hydraulic fluid to the pump for extending and retracting the probe and back up shoe. The reservoir includes a pressure compensator for balancing the pressure in the reservoir to an hydrostatic pressure in the wellbore. A position sensor is coupled to the compensator for determining a position of the compensator in the reservoir, so that a measurement corresponding to a fluid volume in the reservoir can be made. The measurement corresponding to the fluid volume also corresponds to an amount by which the back up shoe and probe are extended outward from the instrument. The amount of extension corresponds to the internal diameter of the wellbore at the location of the probe and back up shoe.
In the method of the invention, the volume of fluid in the reservoir is measured, at the selected depth at which the instrument is to be xe2x80x9csetxe2x80x9d for taking a fluid sample, when the shoe and probe on the instrument are fully retracted. The shoe and probe are then extended, and the volume of fluid in the reservoir is measured again. The fluid volume corresponds to the amount of extension of the probe and shoe.